[187] During the 1950s,
interest in high-energy propellants for upper stages of rocket
vehicles had steadily mounted. Such propellants were initially seen
as a means for increasing the range of ballistic missiles, but this
shifted early towards increasing the capability of rockets to launch
satellites and space probes. The principal candidates were
hydrazine-fluorine, hydrogen-oxygen, and hydrogen-fluorine, but none
had reached the development stage. The coming of Sputnik and U.S.
plans for a strong space program quickly produced action, the first
high-energy upper stage authorized for development used liquid
hydrogen-oxygen. The decision, and subsequent development, owed much
to an earlier program, which had faltered-Suntan, the program to
develop a high-altitude reconnaissance airplane fueled with liquid
hydrogen (chap.
8).